1. Field of the Invention
The present invention relates to a dielectric resonator, a dielectric filter, and a dielectric duplexer, which include a dielectric block and conductive layers serving as electrodes formed on the inner and outer surfaces of the dielectric block, and also to a communication device using at least one of the dielectric resonator, the dielectric filter, and the dielectric duplexer.
2. Description of the Related Art
A typical dielectric resonator for use in the microwave band is formed using a rectangular or cylindrical dielectric block having a coaxial through-hole wherein an inner conductor is formed on the inner surface of the through-hole and an outer conductor is formed on the outer surface of the dielectric block. It is also known in the art to construct a dielectric filter or a dielectric duplexer having a plurality of resonator stages by forming a plurality of through-holes in a rectangular dielectric block and forming inner conductors on the inner surfaces of the respective through-holes thereby forming a plurality of dielectric resonators in the single dielectric block.
Devices such as the dielectric resonator and the dielectric filter constructed by forming conductive films serving as electrodes on the inner and outer surfaces of a dielectric block have the advantages that the total size is small and high unloaded Q (Qo) is obtained.
However, when this type of device is used in a circuit which deals with rather high power, as is the case with a transmission filter or a dielectric duplexer used as an antenna duplexer, it is desired to further reduce the loss of the dielectric resonator or the insertion loss of the dielectric filter so as to meet the requirements of reducing the size and power consumption of electronic devices.
Thus, the present invention provides a dielectric resonator, a dielectric filter, and a dielectric duplexer, which are small in size and have reduced loss.
In general, the loss in a dielectric resonator includes conductor losses in conductive films such as an inner conductor and an outer conductor, a dielectric loss in a dielectric material, and a radiation loss due to energy radiated to the outside. Of these losses, the conductor loss is dominant. Therefore, the key point for reducing losses in dielectric resonators is to reduce the conductive loss.
To reduce the conductor loss, it is effective to form electrodes using a material having high conductivity and to increase the film thickness of the electrodes. However, at high frequencies such as microwave-band frequencies, the current is concentrated by the skin effect in a surface region with a skin depth dependent upon the operating frequency. Therefore, the increase in the thickness of the conductive film beyond the skin depth results in substantially no further reduction in the conductor loss.
If the size of the dielectric block is increased, and if a dielectric material having a small dielectric constant is employed to form the dielectric block, the conductive films will have a reduced current density, and thus the conductive loss will be reduced. However, this technique cannot meet the requirement of reducing the size of the resonator.
In view of the above, the present invention provides a dielectric resonator comprising a dielectric block, an inner conductor formed on the inner surface of a through-hole extending from one end face to the opposite end face of the dielectric block, and an outer conductor formed on the outer surface of the dielectric block, wherein at least a part of at least one of the inner conductor and the outer conductor has a thin-film multilayer electrode structure formed by alternately disposing thin-film conductive layers with a thickness smaller than the skin depth at the operating frequency and thin-film dielectric layers with a particular dielectric constant, thereby allowing currents to be passed substantially equally through the respective thin-film conductive layers of the thin-film multilayer electrodes and thus achieving an increase in the effective area (effective cross section) of the respective current paths and a reduction in the total conductor loss. As a result, a dielectric resonator with a low loss is achieved.
The present invention also provides a dielectric filter comprising the dielectric block described above and external terminals serving as high frequency signal input/output terminals. Herein, the dielectric block preferably includes a plurality of through-holes, and the inner conductors formed on the inner surfaces of the through-holes preferably have the thin-film multilayer electrode structure at locations where they are closest to each other. In this structure at locations where they are closest to each other, the thin-film multilayer electrodes are provided at locations where the electric field is concentrated in the odd mode of the coupling modes of the two resonators, thereby efficiently improving the insertion loss of the dielectric filter.
The present invention also provides a dielectric duplexer comprising the dielectric block described above, an external terminal for connection with an antenna, an external terminal for connection with a receiving circuit, and an external terminal for connection with a transmitting circuit, wherein the external terminals are disposed on the outer surface of the dielectric block. This dielectric duplexer using the single dielectric block may be employed, for example, as an antenna duplexer having a transmission filter and a reception filter.
The present invention also provides a communication device including the above-described dielectric filter serving, for example, as a transmission/reception signal band-pass filter or including the above-described dielectric duplexer serving as an antenna duplexer. Thus, a communication device having a small size and having a high power efficiency can be realized.